1. Technical Field
The present invention relates to a toner, a method for forming an image, and an image forming apparatus.
2. Related Art
There have been image forming apparatuses that use a method including steps of rotatably mounting a photo-conductor such as a photosensitive drum or a photosensitive belt that is a latent image carrying unit onto a main body of an image forming apparatus, and during the image forming operation, forming an electrostatic latent image on a photosensitive layer of the photo-conductor, making the latent image visible in a contact or noncontact manner using toner, and directly transferring the visible image to a material to be transferred through corona transfer or using a transfer roller; or a method including steps of temporarily transferring the visible image to an intermediate transfer medium such as a transfer drum or a transfer belt and transferring the visible image to a material to be transferred again. In these image forming apparatuses, a two-component toner is publicly known, which can provide relatively stable development. However, the mixing ratio of a developer to a magnetic carrier easily varies, which requires the maintenance. On the other hand, a single-component magnetic toner cannot provide a clear color image because of the opacity of magnetic materials.
To obtain a high-quality recorded image while the above-described steps are repeated, toner needs to have a high flowability and be uniformly charged. In particular, when noncontact AC development is employed, from the viewpoint of improving flight behavior, the flowability of toner needs to be improved to decrease the adhesion of the toner to a developing roller. Furthermore, from the viewpoint of improving flight behavior under a development electric field, the frictional charge excessively accumulated in the toner needs to be discharged. Regarding a publicly known toner, silica fine particles are externally added thereto as a flow improver. However, since such silica fine particles have a high resistance of 1015 Ω·cm or higher, a charge-up phenomenon occurs when silica fine particles are charged. Consequently, the repetition of the image forming steps reduces image concentration.
Thus, it is attempted that alumina fine particles are externally added to toner base particles to produce a weak charge-leaking effect. With the charge-leaking effect, the triboelectrification of toner may be stabilized by discharging the frictional charge excessively accumulated in the toner and the flight behavior under a development electric field may be improved. Furthermore, with the polishing effect, the surface of a photo-conductor may be refreshed to stabilize the charge properties of the photo-conductor.
Nano-size alumina fine particles can be manufactured by (1) a low soda method (Showa Denko K.K. and Pacific Rundum Co., Ltd.), (2) a dawsonite method (TAIMEI CHEMICALS Co., Ltd. and HINOMOTO KENMAZAI Co., Ltd.), (3) a spark discharge method (Iwatani Chemical Industry Co., Ltd.), and (4) a flame hydrolysis method (NIPPON AEROSIL Co., Ltd.). Alumina fine particles have various forms such as α-alumina, γ-alumina, θ-alumina, and a mixed form thereof. For example, since α-alumina fine particles have a definite crystalline structure, an oxygen defect (lattice defect) that causes a charge-leaking effect is not easily formed, whereby triboelectrification is not sufficiently stabilized. Furthermore, α-alumina fine particles have a large particle size and a high hardness, which excessively produces a polishing effect. As a result, the excessively polished portions formed on the surface of a photo-conductor cause image defects, and α-alumina fine particles excessively scrape off the photo-conductor layer, which shortens the life of the photo-conductor.
There are proposed γ-alumina fine particles, alumina fine particles coated with silicone oil, and alumina fine particles subjected to surface treatment with a coupling agent (refer to JP:A-3-191363, JP-A-3-240068, and JP-A-8-184988). For γ-alumina that represents transition alumina, an oxygen defect (lattice defect) is easily formed depending on the manufacturing method and a large amount of water of constitution chemically adsorbed to an activated Al—OH group on the surface of the particles is contained, which easily produces a charge-leaking effect. However, it becomes difficult to control a decrease in frictional charge because of the excessive leaking effect and the environmental stability that depends on the water content in the air is degraded. Furthermore, the particle size of γ-alumina can be decreased, but γ-alumina easily forms secondary aggregates due to the activated Al—OH group on the surface of the particles. When γ-alumina is processed so as to adhere to the surface of toner base particles, an alumina free external additive that is present in a maldistribution state causes the following various problems.
Firstly, the leading edge of the triboelectrification characteristic curve decreases as a large number of sheets are printed. In particular, fogging after toner supply is caused in a toner supply type developing apparatus. For the toner in a developing apparatus, when developing operations are repeatedly performed many times, alumina fine particles as an external additive having a charge-leaking effect are gradually lost from the surface of the toner because the alumina fine particles become buried between toner base particles or liberated, which decreases the leading edge of the triboelectrification characteristic curve. (1) When the developing apparatus is a toner supply-type developing apparatus that can be resupplied with toner, a new toner to be used for development is supplied in addition to a residual toner. Alternatively, (2) when the developing apparatus is a single-use toner-type developing apparatus that cannot be resupplied with toner, a new toner is loaded so as to provide a recycled developing apparatus in addition to a residual toner. In either case, there is a difference in electrification capacity during triboelectrification between the new toner and the degraded toner that has been damaged in the developing apparatus. In single-component development, the difference in electrification capacity during triboelectrification between toner and a developing roller that is a toner carrying unit causes problems such as leakage of a toner layer during regulation, scattering of a toner layer on a developing roller at an upper seal, and fogging on a photo-conductor at a portion where an image is not formed, when an image is formed after a new toner is resupplied or loaded. In two-component development, the difference in electrification capacity between the new toner and the degraded toner during triboelectrification between the toner and the toner carrying unit causes problems such as scattering of a developer layer on a magnetic roller during regulation and fogging on a photo-conductor at a portion where an image is not formed.
The mechanism of this phenomenon is described below. Under the circumstances under which the degraded toner and the new toner are contained together in the developing apparatus, the new toner having a superior leading edge of the triboelectrification characteristic curve is formed on the lower side of the toner layer that is formed on a toner particle carrying roller (developing roller) and the degraded toner having an inferior leading edge of the triboelectrification characteristic curve is formed on the upper side of the toner layer so as to be separated from each other. Consequently, the degraded toner formed on the upper side and having inferior charge properties is easily detached from the toner carrying unit during the developing operation.
The toner particles are carried on the surface of the developing roller and pressed by a layer thickness regulating member, whereby the toner particles are rubbed by the surface subjected to a pressing force, the layer thickness regulating member, and the like and charged. The developing roller may have minute projections and depressions on a toner carrying surface by being subjected to sandblasting. However, the size, depth, shape, and arrangement of the depressions are nonuniform. Thus, toner particles that have entered deep depressions are sometimes not rolled and thus not appropriately charged. The nonuniformity of projections and depressions on the surface of the developing roller may locally cause poor electrification of the toner particles. If the toner particles become stuck in the minute depressions, filming may be caused. If the toner particles are not charged appropriately; the toner particles may leak out from the developing apparatus and be scattered in an image forming apparatus or fogging may occur on an image.
In recent years, there has been concern that dust is contained in a cooling airflow exhausted from electrophotographic image forming apparatuses to the outside, and the dust adversely affects the human body. An example of a standard that regulates dust in the air includes a standard regarding fine particulate matter (PM 2.5), which is reviewed by the Ministry of the Environment. In the near future, it is planned for legal guidelines to be disclosed as an environmental standard. It is expected that one of the causes of dust generation is that an external additive having charge-leaking properties is liberated from the surface of toner and emitted to the outside of the image forming apparatus during its operation. Furthermore, from the viewpoint of achieving clearness of an image, the particle size of toner has been decreased in recent years. It is believed that toner having a small volume-average particle size of about 5 μm becomes mainstream particularly in a color image forming apparatus. However, in the system in which an image is formed by applying an alternating current (AC) electric field between the developing roller and the photo-conductor, toner particles move onto the photo-conductor while reciprocate under a development electric field. Therefore, there is also concern that part of the toner activated in a cloud form under the development electric field rides an airflow that flows in the image forming apparatus, whereby the toner becomes dust.
Secondly, a phenomenon of photo-conductor memory is caused. The alumina fine particles as an external additive supposed to move onto the photo-conductor in synchronization with the toner during the developing operation partly move onto a transfer body in synchronization with the toner during the transferring operation, but part of the alumina fine particles in the free state is easily left on the photo-conductor without moving, onto the transfer body in synchronization with the toner. The alumina fine particles left on the photo-conductor are partly removed from the photo-conductor during the cleaning operation. However, a large amount of alumina fine particles remains left on the photo-conductor because the size is much smaller than that of the toner. The alumina fine particles left on the photo-conductor tend to gather at an edge portion of an image when an electrostatic latent image is formed. The alumina fine particles left without being removed in a cleaning step inhibit the operation such as uniform charging or writing exposure. The history (photo-conductor memory) of a change in surface potential becomes a one-step-earlier image forming history when an image is formed, which provides an afterimage.
For toner external-additive alumina fine particles, JP-A-3-191363 discloses a hydrophobic γ-phase alumina polishing material, but does not describe the solution for the above-described problem about charge properties caused by an alumina free external additive. JP-A-3-240068 discloses toner external-additive alumina fine particles manufactured using ammonium dawsonite and JP-A-8-184988 discloses toner external-additive alumina fine particles obtained by adopting amorphous alumina fine particles, both of which do not describe the solutions for the above-described problems.